(1) Field of the Invention
The present invention relates to an image forming apparatus that performs a print operation by thermally fixing an unfixed image formed on a recording sheet, such as a printer and a copying machine. The present invention relates particularly to an art of controlling a fixing temperature for thermal fixing for the purpose of energy saving.
(2) Related Art
Image forming apparatuses such as printers and copy machines perform a print operation by forming a toner image on a recording sheet using page description language (PDL) data, and thermally fixing the toner image. The PDL data prescribes image data and print conditions of the image data.
A fixing temperature that is necessary for thermally fixing toner images differs depending on conditions such as amount of toner deposited on recording sheets and the type of image formed on recording sheets. Accordingly, in order to avoid fixing failure, the fixing temperature is generally set to a target temperature that ensures an excellent fixability even under print conditions that the largest heat amount for thermal fixing is necessary.
On the other hand, from the standpoint of energy saving for reducing electrical power consumption of image forming apparatuses, thermal fixing of all images at the target temperature results in excessive consumption of electrical power. This is not desirable.
As an art of reducing electrical power consumption used in thermal fixing such as described above, Patent Literature 1 (Japanese Patent Application Publication No. 2008-268784) discloses for example an art of acquiring the number of dots in an image region to calculate a printing rate of the image region as an index of an amount of deposited toner, and controlling a fixing temperature for thermal fixing based on the calculated printing rate.
According to this art, when an image has a high printing rate and needs a large heat amount for thermal fixing to ensure fixability, the fixing temperature is set high, for example set to the above target temperature, and thereby to avoid fixing failure. Also, when an image has a low printing rate and needs not so a large heating amount for thermal fixing to ensure fixability, the fixing temperature is set lower than the target temperature, and thereby to reduce electrical power consumption during thermal fixing.
As a result, it is possible to ensure fixability and reduce electrical power consumption, and thereby to realize energy saving.
According to the above conventional art, however, rasterizing process needs to be performed on image data described in the PDL to acquire the number of dots. This takes it a long time to set the fixing temperature of an image to be printed, and a start time of controlling the fixing temperature is delayed. As a result, start of print operation is delayed and productivity of print operation decreases. Especially, print operation of a large amount of print is greatly influenced by decrease in productivity of print operation.